A UMTS communication system, defined in the UMTS standard comprises at least a node B and a mobile phone terminal, referred to as user equipment (UE). Both the node B and the UE includes a transmitter and a receiver. The node B sends signals by means of the node B transmitter to the UE receiver in the downlink. The UE sends signals by means of the UE transmitter to the node B receiver in the uplink. Several UE may communicate with a single node B. In this case the UE are said to be in the same cell. A cell represents the geographical area where a UE is served by a given node B.
In uplink and downlink communication both the UE and node B map the information and control data to be transmitted into logical channels. In particular, the UMTS standard provides for the downlink communication a logical channel denoted acquisition indication channel (AICH). When a UE attempts to establish a communication with a new node B, i.e. to join a new cell, it requests permission to be part of it. The standard procedure for this purpose, in which the AICH is employed, is called random access.
During random access, in order to request access to a cell, the UE sends a short information burst, called preamble, which comprises a signature in the form of Hadamard codes of length 16. Then, the UE waits a lapse in order to receive the response from the target node B. This lapse is called guard period. If the node B receives the preamble without errors, it responds with another burst called acquisition indication (AI).
If no AI is received by the UE after a pre-defined time, a new preamble is sent by the UE. In case of no answer from the node B, the UE re-sends with increased power the preamble up to a maximum number of times. In the case where the maximum number of preamble re-transmissions is reached and no successful AI reception is achieved, the UE signals this failure of the random access procedure to the higher network layers.
The AICH carries the node B messages, i.e. the AI, acknowledging all UE attempting to join a certain cell via random access requests. The AI are mapped on a ternary alphabet, which means that an AI can take on the values 1, 0, and −1. An AI is set to 1 or to −1 to represent a positive or a negative acknowledgement, respectively, from the Node B. If the Node B is unable to provide the service required by the UE then AI shall be set to 0.
All AI are transmitted by the node B on one single channel, the AICH and all UE receive all these AI that are intended for all UE. This means that each UE has to decode all information in all AI, even those that are not intended for that particular UE. A problem is then how to perform efficient decoding of the information in the AI.
According to prior art, standard block codes decoding techniques (e.g. J. G. Proakis, “Digital communications”, McGraw Hill, 3rd Ed. p.436-460) well known to the person skilled in the art, are unable of performing effective decoding of the AI.